Method of operating a continuous filter system



R. N. GILES May 25, 1937.

METHOD OF OPERATING A CONTINUOUS FILTER SYSTEM Filed MarchV l, l1934 2 Sheets-Sheet l Hmm www

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R. N. GILES METHOD OF OPERATING A CONTINUOUS FILTER SYSTEM Filed March l, 1954 2 Sheets-Smet 2 Zow back popczfga cake 597% g3] 74, n y

INVENTOR g5/72. Giles ATTORNEY Patented May 25, 1937 PATENT OFFICE METnoli oF oPEnaTrNG A CONTINUOUS e FmTEit SYSTEM Roy N. Giles, Hammond, Ind., assignor to .Stand--A of Indiana ard Oil Company, Chicago, Ill., a corporation` Application March 1, 194, Serial No. '713,484

a claims. (el.` 19a-19) The principal object of my invention is to pro vide a practicalV and elcient method of operating `a continuous process for filtering wax-bearing oils in very light solvents. The methods of filtering wax from wax-bearing oils dissolved ln light solvents which have been generally in use heretofore are intermittent, in that the press soon becomes filled with wax whichl must be removed. The frequent cleaning of the presses entails loss of time andincreases the expense ofv operation. Various types of apparatus for the continuous filtering of-waxy oils'have been devised but have not provedsatisfactory for very light `solvents when operated.

25 one side and that of the clarified filtrate on the lother. 'I'he liquid phase inthe slurry is of necessity saturated with wax. Therefore evaporation of solvent from the liquid phaseupony passage o through the filter medium must cause precipitation of wax. -This precipitation takes place in the fiber of the fabric and soon renders it progressivelymorc impervious to the passage `oi liquid.

The filter rate is correspondingly cut down and 35 the eiliciency of the lter lowered until no ap/ preciable filtration takes place. f

I have invented a method of operating continuous filters which prevents the above /described diiiiculties from taking place. My method o operates without the complication of a gaseoussubstance other than vapor of the solvent itself. In this method the slurry, which has been independently chilled to the desired temperature, is

introduced intol the filter apparatus which will al- 45 low a relatively large ratio of volume to-fsurface area of the slurry.

Pressure is exerted upon the surface of the slurry by the vapors of the solvent. Thispressure is always greater than the vapor pressure 50 ofthe solvent in the slurry. The filter medium is vimmersed in this liquidrand on the low pressure or clear ltrate4 side a pressure ,is maintainedby vapors of the solvent, which pressure isslightly greater than, or atleast equal to, the vapor pres- 55 sure of solvent in the clarified filtrata 'I'he pressure on the slurry is always maintained at a differential of a few pounds only over that of the clear filtrate. This condition of non-equilibrium is maintained by virtue ofthe following: First, the surface area is small compared to theV 5 volume of liquid slurry; second, the lter rate is rapid; third, the condensed solvent forms only 5a surface lm of low gravity which has no tendency to convection, and fourth, the diiusion of liquid from the surface into the body is tooA slow l0 to be a disturbing factor.

The advantage of a continuous process when operated by my method are: First, I obtain by continuous .filtration a muchhigher lter rate than can be realized by'present types of presses l5 second, by having a continuous. removal of the wax cake I am able to handle any type of wax cake which therefore permits me to dewax any type of stock; third, I am able to obtain a high yield of oil and fourth, the washing of the cake is veryefflcient.- Also my method entails less voperating expense. fewer acts of manipulation p and less investment in tankage, pumps, lines,- etc.,l per unit of electricity. The qualities of the products are in every way equal to or superior to those :of products from batch operation. Further ob- 4jects and advantageswill `be apparent from the following detailed description.

In the accompanying drawings which form a part of this specification and in which similar parts are designated by black reference characters inthe several views,

Figure 1 is a diagrammatic plan of 'my iniproved filter system; y. I

Figure 2 is a diagrammatic cross. sectional view 35 of the vilteraindicating the pressure regulation features; and l Figure 3 is a modied plan of asystem employing a non-condensable blow-back gas.

In describing my invention I will refer to 40 propane as a diluent, but it should be under--` 'stood that this is merely an example of light, normally gaseous diluents and that I may use butane, ethane, propylene, butylene, methyl chloride, methyl ether, etc., or with equivalent substances either alone or in admixture with each other. The present invention is not dependent upon the use of any particular diluent, but relates to an improved method and apparatus for emrploying any normally gaseous or easily condensable diluent.

The invention will be described as applied to an S. A. E. 50 distillate lubricating oil stock prepared` from Salt Creek crude, but it should be understood that the invention is applicable to any wax- 'I'he waxy lubricating oil stock is introduced through line 9 and propane is introduced through `line I0. The oil stock lmay bedissolved in the propane and maybe'fchille'd by auto-refrigeration v or by'any other means in crystallizing plant II as disclosed in my prior Patent No. 1,943,236: The particular method of chilling forms no part of the present invention and it is sufllcient to point out that a filterable propane-oil-wax slurry may be obtained by any known process and introduced into wax slurry feed tank I2, this tank acting as a supply reservoir for the slurry which is charged to the lter. 'I'his slurry preferably contains'about one part of oil to three parts of propane by volume. The slurry is preferably maintained at a temperature of about -40 F. The low temperature may be maintained byremoving propane vapors through pipe I3 and propane suicient to provide this cooling and to supply necessary diluent may be introduced through line I4 from a cooled propane source.

The filterable wax slurry is transferred from tank I2 through line' I5 byspump I6 to the feed bowl I'I of a continuous lter. The slurry is introduced more rapidly than it is filtered and the excess slurryis returned to feed tank I2 through pipe I8 in amounts regulated by valve I 9 which is operated by control 20 actuated by float 2I which in turn is supported on the surface of the slurry in feed bowl I'I. Floats 2IA may be used to protect the surface of the slurry from the warmer blow-back gases as will be hereinafter described.

The continuous filter (see4 Figure 2) may be of any well known type and as a preferred `ex' ample I will describe the use of a cylindrical filter having an insulated gas tight pressure casing or shell 22 enclosing a drum 23 having a filter element 24 on its periphery and provided with segments 25 connected by sliding ports flrst to the filtrate discharge slot 26, then to the wash liquid slot 21, and finally to the blow-back slot 28. Cold wash propane is introduced through pipe 29 and may be sprayed onto the cake by perforated pipes or nozzles 30.

Pressure is maintained between the lter shell and the drum by means of propane gas from line 3l. A portion of this gas is introduced through line 32 as blow-back to facilitate cake discharge, the amount of gas introduced being controlled by pressure regulator 33. Gas is also introduced at a after operation is under way, by pressure con.

trolled valves or the vapor discharge pipes which lead 4from the storage tanks to which pipes 3i and 38 are respectively connected.- The wax cake is removed from the lter element by scraper 4U and conveyed' by scroll conveyor 4I through conduit 42 to a wax pot.

An outstanding feature of my invention is the careful control of pressurein the various parts of the system, so that the evaporation and condensation of propane can be controlled to supply the necessary refrigeration and pressure differentials without-impairing cold test of finished oils, plugging the filter elements, etc. I have discovered that when large pressure differentia... are employed in a system which uses exclusively a condensable gas like propane, the high pressure causes an excessive condensation of propane in the wax slurry and particularly on the wax cake whicl warms up the wax cake, redissolves wax, and carries the Wax into and through the lter element. I have also discovered that if the pressure on the filtrate side is not maintained above the vapor pressure of the filtrate there will be a ashing oi.' diluent from the inner filter surface which will cause a deposition of vNiscous oil and wax in the pores of the filter thereby :clogging the filter and impairing if not actuallystopping the flow of diluted oil or wash liquid therethrough.

In practicing my invention I preferably maintain a small differential, about two to eight pounds, between the outside and inside of the filter element and I maintain the pressure on the inside of the filter element higher than the vapor pressure of the filtrate. Thus if the average temv perature of the slurry is 15 F. and if the theoretical vapor pressure of the slurry is 16 pounds, I Amaintain a pressure inside the rotating drum (P1) of 16.8 pounds and I maintain a pressure between the iilter element andthe lter shell of 20 pounds, giving a filtering pressure differential of 3.2 pounds. Under these conditions a uniform wax cake is formed at all times and filtration can be continued indefinitely giving filter rates upwards of one gallon of dewaxed oil per square foot o f filtering area per hour. If. an attempt is made to operate a system of this type with a pressure inside the drum (P1) less than the vapor pressure of the filtrate, evaporation of diluent will occur both from the ltrate a d from' the inner surface of the filter cloth and filtration will practically cease. y

Although I do not use extremely high propane pressures between the shell and drum. these pressures are higher than the vapor pressures of the slurry and unless there is little or no space between feed bowl Il and drum 23 I prefer to use floats ZIA to cover the surface and prevent the condensation of propane' gas in the wax slurry.

Another important feature of my invention is the utilization of gases in the shell (from the blow-back gas, wax pot, filter slurry, etc.) to effect the final washing of the filter cake.' Since the shell gas is at slightly higher pressure than the vapor pressure of the wax cake composition there-will be a slight condensation of this gas throughout the entire surface of the wax cake which is exposed. This surface condensation takes place in a very short interval of time and the amount of wax dissolved is negligible. The condensation occurring first only on the surface of the wax cake, it is sufficient to drive the last remaining washv liquid out of the wax cake. This use of shell gas to supply wash liquid has been found to greatly improve the vquality of the cake and to facilitate an even, uniform cake discharge. Referring once more to Figure 1, the wax cake -which is forced by scroll 4I through conduit 42 to wax pot 43 is melted in said wax pot bysteam coil 44, the amount of steam being regulated by thermostatically controlled valve 45. Propane vapors are withdrawn through pipe 48 through vpreamike-controlled valve 41 and low pressure gas line 48. Wax is withdrawn from the base of the' pot through pipe 49. An important feature of this arrangementis the countercurrent use of propane 'gas owing back through conduit 42 toward the lter. This propane gas is condensed by the approaching vwax slurry, so that the w x slurry is both warmed and diluted, thereby minimizing the amount of work required by the s/croll.

I n fact, a' portion of the propane between the illter shell and drum may come from this wax scroll system; thermostatically controlled valve t tank 50 from which vapors are vented through 66 to low pressure gas line 4B.

line 5I and pressure-controlled valve 31A to low pressure gas line 48. When the filter is'just beginning to operate it may be necessary to employ pressure control 31, but during the normal operation I prefer to open valve 31 and to employ the pressure control on valve 31A. The cold Illtrate is warmed by a vheat exchange which will be hereinafter described and this warmed Atil-- trate is then passed by means of pipe -52 to suitable.- stills for removing diluent which is condensed and returned to diluent storage in the conventional manner.

Gases from low pressure gas line 48 discharge into the inlet Vside of compressor 53. 'I'his compressor discharges Yto intercooler 54 and the cooled gases from the intercooler are then passed' through compressor 55, line 56 and condenser 51 to propanestorage tank 58. Intercooler 54, between stages of compression lessens compression costs to a considerable extent.

The pressure in propane storage tank 58 is maintained 'wit-hin safe limits by a suitable safety valve in line 59 which may lead to fuel gas lines, absorption systems, compressors, etc. This may be used vto purge out any air that may be drawn into the system.

Propane is passed through liquid line 60 to cooling coils 6I inliltrate receiver '50, so that the refrigeration value of the nitrate may be utilized.

The cooled propane may then be passed through line 62 to cooler 63 which is cooled by the vaporization of propane -introduced through pipe 64, vaporized in coils 65 and released through valve The propane in cold propane tank 63 supplies the propane for washing the lilter through pipe 29 and supplies propane for cooling anddiluting the wax slurry through line I4. I may, of course, effect cooling in tank 63 by venting vapors directly therefrom to low pressure gas line 48 through valve 66A.

The wash nitrate from the iilter is introduced by pipe 38 to storage tank 61 which maybe provided With a heat 'exchanger similar to that described for tank 50. The wash filtrate removed from the base of this tank through line 68 may be combinedwith the ltrate or admixed with incoming lube stock for further dewaxing, but it may, of course, be dewaxed in a separate system l or utilized for-some purpose which does not require as low pour point as the main nitrate. It

should be understood, of course that'some of the washed liquid may beremoved with the nltrate and/or that the wash filtrate maybe separated into various fractions of increasing wax content. This may be effected by adjusting slots 26, 21, etc., as is well known to those skilled in the art.v Vapors may be removed'from wash ltrate storage tank 61 through pressure-controlled valve 39A and pipe 69 to low pressure gas line 48.

' The modication of my invention hereinabove essary to separate the shell `gas from vaporized 3 diluent. By employing one and the same material for diluent and shell gas I have greatlysimlplified' the necessary equipment and by employing the critical pressures hereinabove described I obtain excellent yields of low pour point -oils and I obtain' dry wax cakes which are readily discharged from the filter, so that the entire process may belcarried out in a continuous and very sat` isfactory manner. Using diluent ratios of about 3 to l, lter pressure diierentials of about 3 pounds per square inch; operating the lter at about 1A revolution per minute to form a cake of about 1A; to 1A; inch thickness, I can obtain lter rates of from l gallon to upwards Iof 4`gallons i of oil per square foot of lter area per hour.

Ifhigher pressuresare desired in the filtration step, I have found that my improved apparatus is particularly well adapted for employing anoncondensable shell gas and such a modication ls illustrated in Figure 3. The inert gas, which may be a lighter hydrocarbon such as ethane orf which may be an inert gas such as nitrogen,

rator 14, the liquid propane being returned through pipe 15`to propane storage tank 58 and the gases being recycled through line 16 to line 3l and thence to the filter. Excess pressure in separator 14 `may be released through line 11. In some instances it may be unnecessary to employ a liquid separator at all and the compressed gases may be merely cooled in exchanger 18 and returned to the ilter. I .preferfhoweven to employ the liquid separator because such a separator acts. as a surge tank to insure a-uniform pressure on the shell gases and it provides means for proportioning the amount of propane vapors which are returned to the filter with the shell gas. Thus by controlling the temperature in separator 14 I can return just enough propane withthe shell gases to insure that the partial pressure ofpropane in the space between the lter shell and the iilter drum is exactly the same as the partial pressure of the propane in the wax cake. In other words, I can use high filtering pressures without causing an excessive amount of condensation' of propane on the filter cake or a large amount of propane vaporization from the iilter cake. This is an extremely delicate procedure because excess propane condensation causes a resolution of wax, high pour point ltrates, clogged lter's, etc. and a nexcess vaporization of propane causes disruption of the lter cake, clogging of the lter element, etc.

While I have described in detail the dewaxing of S. A. E. 50 Salt Creek distillate stock, it should be understood that the invention is equally applicablef to other waxy oils including S. A. E. l0

and 20 stocks and even bright stocks or cylinder.

stocks. The amount of diluent propane required may vary with variousstocks, but the optimum quantities for lterable mixes may be readily determined by preliminary tests. An important feature of my invention ls the maintenance of a pressure on the flltrate'side of the filter element which is at least equal to the vapor pressure thereof, so-that there will be no ilashing or boiling of the ltrate or propane from the inner side invention to employ a'relatively small pressure difrerentialwhich is preferably about two to eight of the lter element. It is also a feature of my or perhaps ten pounds inthe ,case of propane. Of course, this pressure diierential may be increased if an inert, non-condensable shell gas is employed and'in that case I prefer to maintain the partial pressure of propane at about the same pressure as the vapor pressure of the wax cake mixture.

While I have described in detail a. preferred embodiment of my invention it should be understood that I do not limit myself to any of the details hereinabove set forth except as defined by the following claims which should be construed as broadly as the prior art will permit.

Iclaim:

1. The method of separating wax from oil which comprises chilling a mixture of waxy oil and propane to solidify the wax and form a filterable oil-wax slurry diluted with propane, introducing said slurry to the feed bowl of a continuous filter, maintaining the pressure on the ltrate side of the continuous lter higher than the vapor pressure of the ltrate, continuously removing wax from the filter, heating and melting the wax l at a point remote from said filter whereby diluent 40 stantially the same composition as said diluent,

is vaporized, passing said diluent vapors countercurrent to said wax leaving the filter whereby the wax is warmed and the diluentkis cooled, and returning said cooled diluent to said lter in contact with wax which is being removed.

2. The method of dewaxing oils in a system employing a continuous filter containing a rotary' partially immersed filter element, which method comprises chilling a solution of wax-containing oil in a normally gaseous diluent to solidify the wax in readily lterable form and to make a.

cold slurry thereof in the liquefied normally gaseous diluent, introducing said cold slurry into said filter, simultaneously introducing as a pressuring iiuid into said filter a condensable gas of subwhereby a single iiuid composition serves both as diluent and pressuring gas, and whereby the condensation of portions of the pressuring gas provides wash liquid on the non-immersed portion of the lter cake.

3. 'I'he method of claim 2 wherein the pressure on the filtrate side of the lter element is maintained above the vapor pressure of the filtrate.

4. The method of claim 2 wherein the pressure on the filtrate side of the filter element is above the vapor pressure of the nitrate and wherein a pressure differential of about 2 to 8 pounds is maintained across theiilter element.

5. The method of dwaxing mineral oils in a system employing a continuous lter containing a rotary partially immersed filter element, which method comprises chilling a solution of waxbearing oil in propane to solidify the wax in iilterable form and to make a cold slurry thereof in the propane-oil solution, introducing said cold slurry into said filter, simultaneously introducing propane gas as a pressuring uid, whereby propane serves both as a diluent` and pressuring gas, and whereby the condensation of portions of the propane provides wash liquid on the nonimmersed portion of the lter element.

6. The method of claim 5 which includes the step of maintaining a pressure on the filtrate side of the iilter element which is greater' than the vapor pressure of the filtrate.

7. The method of claim 5 which includes the steps 'of maintaining a pressure on the filtrate side of the filter element which is greater than the vapor pressure of the filtrate, and maintaining a pressure differential across the lter element of about 2 to 'pounds per square inch.

8. The method of claim 5 wherein at least a part of the propane pressuring gas is introduced to the filter in direct contact with wax which is being removed from said lter.

ROY N. GILES. 

